Internal combustion engines are in widespread use and are used to power crafts and vehicles of different sizes ranging from small radio controlled aeroplanes to large ocean going vessels such as oil tankers. It is therefore not surprising that internal combustion engines are constructed using a wide variety of different configurations which typically used to classify the engine. Common configurations include two or four strokes and a Wankel engine (also commonly called a rotary engine) although other configurations exist such as using five- and six-cycles, a diesel cycle or a Brayton cycle.
A primary concern in engine design is improving the power-to-weight ratio of the engine. For example, although Wärtsilä RTA96-C 14-cylinder two-stroke Turbo Diesel engine produces a peak power output of 80,080 kW, due to the size of the engine the power-to-weight ratio of the engine is only 0.03 kW/kg. A marginally better power-to-weight ratio is produced by a Suzuki 538cc V2 4-stroke gas (petrol) outboard Otto engine which has a peak power output of 19 kW resulting in a power-to-weight ratio of only 0.27 kW/kg. A Wankel engine configuration achieves a better power-to-weight ratio of 1.15 kW/kg from a 184 kW engine. BMW has achieved a power-to-weight ratio of 7.5 kW/kg with their 690 kW BMW V10 3L P84/5 2005 gas (petrol) Otto engine. Is therefore clear that different engine configurations achieve different power-to-weight results and that a balance must be struck between achieving a desired amount of kilowatts on the one hand and the weight of the engine on the other hand.
A commonly used configuration in motorised road vehicles is the four-stroke or Otto design. Typically such an engine has four strokes from one combustion stroke to the next. An air mixture containing a flammable liquid such as high octane petroleum is compressed inside a piston-cylinder assembly. This compressed air mixture is ignited at a predetermined time thereby causing in the combustion stroke the piston to move away from a cylinder head of the piston-cylinder assembly. This linear movement of the piston is transferred through a crank to one or more wheels of the vehicle through a drive train or gearbox. Although typically such an engine has a sufficient power-to-weight ratio for use with a vehicle, it is often required to improve this power-to-weight to increase the fuel efficiency of the vehicle.
Otto engines normally deliver a maximum amount of torque at high revolutions which, when the engine is often revved to a high revolution, could result in reducing the life span of the engine. This may be undesirable.
A further aspect which greatly determines the live span of an engine is the configuration on which the engine is based. In an Otto design engine the piston travels four times along the length of the cylinder from one compression stroke to the next. Accordingly, such engines will therefore have a shorter life span than an engine which is based on a configuration using fewer strokes, for example a two-stroke engine.
Often an engine incorporates more than one piston irrespective of its configuration. Due to the mechanical forces operating inside the engine, it is critical that the engine is balanced as far as possible. As a result, engines ordinarily include an even number of pistons thereby allowing the number of pistons to be grouped in smaller groups each having an even number of pistons. This allows the smaller groups of pistons to move in unison and preferably in an opposite direction than another small group of pistons. However, the use of smaller groups of pistons may still cause the engine to become unbalanced.
In conventional engine configurations linear movement of the pistons are converted into rotational movement by the crank to which the pistons are connected. This connection typically requires a connection rod extending between a respective crank pin and piston to move, apart from linearly, also from side to side. The side to side movement, although being partly accommodated and countered by the crank and the flywheel, nonetheless causes some unbalancing of the engine and increases stresses being placed on other moving components of the engine. It therefore can be desirable to improve the balance with which components move inside the engine and thereby reducing stresses placed on moving components inside the engine.